110 M ODEL AVIATION
a fRequenT question concerns the benefits of purchasing a
higher-end computer radio.
Medium- and higher-priced computer radios offer features that
can greatly enhance flight performance. The purpose of this month’s
column is to examine some features of better computer radios and
their application to Scale Aerobatics.
The majority of Scale Aerobatics airplanes are comparatively
simple: one engine, two elevators, two ailerons, and one rudder.
Smoke is about the only option.
Many designs (including Giant Scale) can be flown using a
lower-end four-channel analog radio. A lower-end radio will suffice
for a sport modeler who isn’t interested in expanded capabilities or
advanced setup, but there are a number of features on computer
radios that will literally change the “feel” and characteristics of
almost any Scale Aerobatic airplane.
Some features, such as exponential, have the same effect whether
the airplane is a P-51 Mustang or an Extra 300. Other features, such
as programmable mixing, may be used on a P-51, but for a different
purpose than on an Extra.
exponential: This is arguably one of the most useful features in
achieving a good setup. Exponential is defined as the mathematical
relationship between the stick movement and servo travel.
If servo travel was linear, a radio without exponential would
exhibit a proportional movement of how far the stick was deflected
(50%, for example) and the travel of the servo.
However, the relationship is inverse because servo arms move
in an arc, meaning that the throw decreases as the stick travel
increases. (Some modelers use large control arms to overcome the
loss of travel.)
Without exponential the stick/servo relationship at center is close
to linear, meaning that a slight movement in the stick will cause a
similar movement in the control surface.
For an aerobatic airplane, the result is a model that is very pitchsensitive,
a handful to land, and not much fun to fly. The purpose of
Paul Kopp, 1013 S. Sedona Ln., Anaheim CA 92808
RADIO CONTROL SCALE AEROBATICS
2000 Tournament of Champions winner Christophe Paysant Le roux
demonstrates a low smoke-on hover with his ZNLine Extra 330.
District X AVP Bill Malvey with Midwest CAP 232 (BME 2.7, Menz
20 x 8). He flies with the Southern California Aerobatic Team.
Kevin Lips, sitting on the left, discusses the finer points of
judging with Jerry Bien of Airwild model products.
May 2001 111
an exponential function is to “dull” the center of the stick by
changing the stick/servo-travel relationship.
The pilot “feels” an airplane that is much less sensitive near neutral,
making it much easier to control. How much exponential is needed
depends on the airplane and is a matter of personal preference.
Knife-edge mixing: Many Scale Aerobatics airplanes will hold
sustained knife-edge. However, very few airplanes will hold knifeedge
without pitch and/or roll corrections.
When most airplanes are rolled into knife-edge (using rudder only),
they will probably not fly straight. Many will “push” into the belly
(midwing designs may pull toward the canopy), and exhibit a roll
tendency, either out of knife-edge toward the rudder or away from the
rudder. The propensity to pitch or roll from rudder is called “coupling.”
An important concept is that although coupling may be most
pronounced in knife-edge, it is inherent in the aircraft design. Some
pilots unknowingly experience pitch coupling when they make a
rudder correction during landing and the airplane’s nose drops
suddenly. No, it wasn’t a radio hit.
The ability to hold knife-edge depends (among other things) on
holding the required corrections. For simple knife-edge from one
end of the runway to the other, holding the corrections requires fixed
inputs. Try to do a long, slow roll or opposing knife-edge, and it’s a
different story. The airplane may wander and change roll rate.
When I first started flying competitively, I was using a Carl
Goldberg Models Sukhoi with an older analog radio with no mixing.
Because I couldn’t correct the pitching in knife-edge, I had to do
slow rolls by pitching the nose up and flying through the maneuver
in an arc, with little or no rudder. Those rolls were really ugly!
It became a different airplane with rudder/elevator mixing—slow
rolls were straight and axial.
Programmable mixes are used to minimize or eliminate
coupling. Mixing allows the modeler to create a relationship
between two channels: one designated the “Master” and the other
the “Slave.”
(“P mixes” are significantly different from lower-priced radios
that contain dedicated mixes, such as Aileron/Rudder. Those are fine
for a Cub, but opposite of what we need.)
A programmable mix allows the modeler to create elevator or
aileron corrections. The rudder channel is the Master and the
elevator or ailerons, the Slave.
The “program” is creating the relationship between the
independent channels and the amount of deflection for each surface.
Both are done in the same program. If the airplane has pitch and roll
coupling, it will require two independent mixes.
Now take this one step further: suppose you have two elevator
servos and you want to run them on separate channels.
The radio will need to have a function that allows both of the
elevators to be controlled by the same trim. That feature is found only
on medium- to higher-end radios. (Without it, you can set up a dualelevator
mix, but only one side will respond to the elevator trim.)
Assuming the radio has dual elevator/trim capability, the next
step is to establish the Master/Slave relationship described earlier.
For example, the rudder channel remains the Master and the
primary elevator channel becomes the Slave. The movement of
the primary elevator will drive the second elevator if the
elevators are mixed. When the rudder stick is deflected, both
elevators will respond.
multipoint mixing: High-end computer radios provide additional
mixing capability, known as multipoint mixing.
The programmable mixing on medium-priced radios relies on a
linear relationship between the Master and Slave channels. If an
elevator has a mix value of 10%, as the rudder is deflected the
elevator will gradually deflect until the 10% maximum is achieved.
Multipoint mixes allow modelers to change the mix relationship
at different points, thereby creating a mixing “curve.”
Multipoint mixes are useful when the coupling is not linear. We
may establish “uncoupled” knife-edge flight, meaning that by using
the linear mixes, the airplane can fly knife-edge in either direction
using only rudder inputs.
But suppose we want to do knife-edge loops. The extra rudder
deflection may require mixing corrections in excess of what is
provided with a linear mix. A multipoint mix allows the modeler to
provide the additional corrections without disturbing the mix needed
for level knife-edge.
condition switches/Flight Modes: Condition switches, or flight
modes, are found on high-end radios. With the increasing interest in
3-D flying, this feature has recently attracted many users other than
high-level competitors.
The benefit is that the airplane’s setup can be changed
instantly from precision, utilizing low, highly moderated
throws, to 3-D, using extreme throws with different
exponential curves, servo throw, and mixing.
It is a very useful feature—especially among pilots who fly in
precision and freestyle competitions.
programming hints: There is one hint for programming, regardless
of a pilot’s experience: do it with the airplane on.
One pilot couldn’t figure out why his Extra 300 pitched to the
belly even after he set up what he thought was the correcting mix.
What he didn’t check was that the elevator “correction” was going
in the wrong direction.
Exponential has the same caveat: it’s very easy to create
“negative exponential” (increasing the sensitivity near center) by
going the wrong way on the value.
To ensure that you are going the right way, go to an extremely
high value and watch the control surface move. If the surface is
unusually sensitive around neutral and dead at the end, you may
have accidentally created negative exponential.
Setting up knife-edge mixes is trial-and-error. Because of
differences in the centers of gravity and other setup discrepancies,
two airplanes may require different correction percentages.
Starting at zero, roll the airplane into knife-edge and gently
feed in enough rudder to hold altitude while holding the required
Jason Shulman’s all-composite FiberClassics Extra 330XS with
DA-150 engine. Jason took third at the 2000 TOC.
District X contest coordinator Tim Attaway (Chula Vista CA) with
a very nicely finished Midwest Extra 300S.
112 M ODEL AVIATION
corrections. Have a helper write down
the corrections by direction (such as
right to left, up-elevator, right aileron).
Correct one surface at a time,
making sure that the corrections are
visually in the right direction. When
the mixes are properly set up, the
airplane should remain in straight,
level knife-edge with rudder only.
A number of other features distinguish
computer radios; however, it is the mixing
capabilities that are most often used for
Scale Aerobatics airplanes.
Mixing can also be used to turn
smoke pumps on and off, create
engine kill switches (a nice safety
feature), and cure minor pitching
tendencies in vertical lines.
Rcs 1.4: RC Showcase (RCS) recently
released a 1.4 (23cc) gas engine, designed to
replace 1.2 glow engines. I was lucky
enough to get one to review, and I have been
running it in a couple different airplanes.
The engine is turning in the mid-
8,000s with an APC 16 x 8 propeller,
making it comparable to a good 1.2.
Unlike other small gassers, the RCS is
extremely light—another important
consideration.
RCS has a full complement of
accessories for the 1.4, including a
predrilled Du-Bro soft mount, an ignition
switch, and a muffler. MA
Midwest
Products
Co., Inc
K
Kit - Sport
Film
provided
envelope
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